Direct evidence for distinctive, stress-induced nucleus crystals in the crystallization of oriented polymer melts

A previous paper proposed a model for polymer crystallization under stress [1]. This paper suggested that there could be distinctive crystals which formed under stress at unusually high temperatures to act later as nuclei for the bulk of the crystals. The present paper details definitive evidence for these nucleating crystals. Electron microscope observations are fully consistent with the expected two-phase structure for crystallization under stress, although they are not conclusive. However, X-ray diffraction patterns recorded while the samples crystallized under stress, at the appropriate elevated temperature, conclusively demonstrated the two-stage nature of the crystallization. First, a small number of crystals (type I) form which are highly c-axis oriented, followed by a second group of crystals (type II) having a more complex stress dependence of texture patterns. This is in complete agreement with the model [1] that c-axis-oriented fibers induce transversely growing lamellar crystals. Type I crystals show all the features expected from stress-induced crystallization; thus they melt and reform on relaxing and reapplying the stress, while type II crystals do not. It follows that the small amount of type I crystals support most of the load inastress-crystallized sample. The new experimental approach makes the nucleating crystals accessible to direct examination, some further examples of which are being reported.